It is known that diaryl sulfones can be synthesized by contacting a monosubstituted benzene with a monosubstituted benzene sulfonic acid at elevated temperatures. Also known is that such syntheses generally achieve only partial conversion of the sulfonic acid with the unreacted sulfonic acid being discarded in the recovery of the desired diaryl sulfone product.
For example, in U.S. Pat. No. 2,593,001 to Bender et al., there is described a synthesis method involving the counter-current reaction of monoclorobenzene vapor with benzene sulfonic acid to form p-monochlorodiphenyl sulfone. Though the proposed method is described to be of good yield, the resulting product mixture contains nearly as much unreacted sulfonic acid as diaryl sulfone product.
To recover the diaryl sulfone, Bender teaches washing away the unreacted sulfonic acid with copious amounts of water. This separation method is conventional in industrial practices. Even in processes where sulfonic acid is the principal and desired product, such as the processes described in U.S. Pat. Nos. 3,935,237 and 3,946,037, large amounts of water, generally well in excess of the amount of benzene sulfonic acid produced, are employed to separate the benzene sulfonic acid as an aqueous solution from the diaryl sulfone. However, in using such large amounts of water, the sulfonic acid is rendered practically useless for forming additional diaryl sulfone.
It is essential in forming diaryl sulfones that the reaction medium be maintained substantially anhydrous during the reaction. While amounts of water up to about one percent can be tolerated, amounts of water substantially greater than this are desirably avoided as the presence of water leads to very low reaction rates. Consequently, the aqueous solution of sulfonic acid from the conventional diaryl sulfone recovery cannot be recycled to form additional product due to its large water content.
Further, dehydration of the aqueous sulfonic acid has been economically prohibitive due to the large amounts of water used to recover the desired diaryl sulfone product. Therefore, unreacted sulfonic acid, heretofore, has been treated, as it was described in Bender, as by-product waste.
Present commercial synthesis of diaryl sulfones generally produces sulfonic acid then "captures" it by forming a sulfonyl chloride derivative by reacting thionyl chloride with the sulfonic acid, either as it is produced (e.g. U.S. Pat. Nos. 3,701,806 and 4,172,852) or subsequent to its production (e.g. U.K. Patent No. 1,572,916). The aromatic sulfonyl chloride then is reacted with a monosubstituted benzene in the presence of ferric chloride catalyst to yield the desired diaryl sulfone. These methods of producing diaryl sulfone are sufficiently advantageous and efficient for commercial operations. However, thionyl chloride is a relatively expensive chemical which is lost in the process as sulfur dioxide and hydrochloric acid off-gases.